1. Field of the Invention
This invention relates to detecting arcing faults in electric power systems and particularly within the metal cabinets of switchgear. More specifically, it relates to apparatus and the method for detecting such arcing with optical sensors which are resistant to false indications induced by other light sources such as ambient light, tungsten bulbs, fluorescent lights, flashlights and even flash bulbs.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems which form part of the overall electric power system include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within the switchgear cabinets, such as between the buses, or between a bus and a grounded metal component. Such arcing faults can produce high energy gases which pose a threat to the structure and nearby personnel. A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot be contained.
Recently, methods have been developed for minimizing the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage. The light detection methods used to date are not selective so that any light signal can trigger the sensor. Thus, a flashlight used by service personnel, or sunlight or a photoflash can falsely trigger the optical sensor. In order to avoid such false tripping of the optical sensors, it has been proposed that optical sensing be combined with current sensing either directly or by sensing the magnetic field created by the current. Unfortunately, these methods are costly and have not proven to be reliable.
There is a need, therefore, for improved apparatus and method for detecting arcing faults in electric power systems and particularly within switchgear.
More particularly, there is a need for such improved apparatus and method for detecting arcing faults which respond before significant damage has occurred and yet are reliable and resistant to false arcing fault indications.